A mounting substrate for a fluid ejection device, such as an inkjet printhead, has conventionally been made by a single molding process which forms both the die-attach portion for the fluid ejection device(s), including the fluid feed channels or slots with lands there-between, and a housing portion including alignment and fastening features, such as bolt holes. The mounting substrate should be sufficiently strong that it does not deform during fabrication of the mounting substrate, during attaching of the fluid ejection device(s), during attaching of the mounting substrate to a printhead chassis, or during printhead operation. If the fluid ejection device(s) to be attached to die-attach portion have multiple fluid inlets that are spaced apart by about 2 millimeters or more center-to-center, use of a single molding process provides satisfactory results. Such multiple fluid inlets can, for example, be for providing different colored inks (e.g. cyan, magenta, yellow and black) to an inkjet printhead die having separate arrays of drop ejectors that are fed independently by the fluid inlets.
One way to reduce the cost of an inkjet printhead is to reduce the size of the fluid ejection device, i.e. the printhead die, which typically includes not only the fluid inlets and the arrays of drop ejectors, but also includes logic and switching electronics, as well as electrical interconnections. Due to advances in microelectronic fabrication processes, making the electronics on the die fit within a smaller space is now possible, so that the fluid inlets on the printhead die can be spaced as close together as 0.8 mm center-to-center or closer. The problem that remains is providing a mounting substrate having a die-attach portion with fluid feed slots at the same spacing as the fluid inlet spacing on the printhead die.
It is difficult to make fluid feed slots at a center-to-center spacing of less than one millimeter in a single injection molding process step and still provide sufficient strength in the mounting substrate. This is because for precision single-step injection molding processes, all wall thicknesses need to be substantially uniform. For example, for a center-to-center fluid feed slot spacing of 0.8 mm, the width of the slots and the widths of the lands between the slots can each be about 0.4 mm. This means that all walls that are injection molded in the same step should have approximately the same wall thickness as the lands, i.e. about 0.4 mm. It is found that such thin wall thickness may not provide a sufficiently strong, flat and stable mounting substrate.
Alternatively, if the walls or other features in the rest of mounting substrate were made substantially thicker than the lands between the slots, the molding material would not flow in a uniform manner to fill both the thick walls and the thin lands. As a result, the die-attach surface can warp, so that it is insufficiently flat to allow the printhead die to be adhesively attached with reliable fluid seals between adjacent fluid feed slots. In addition, there can be “knit lines” resulting from molding material flowing from both ends of the fluid feed slot and land region and meeting midway down the lands. Such knit lines are built-in discontinuities and stress concentrations which can lead to deformation and failure in the part.
Commonly assigned US Published Application No. 2008/0149024 (incorporated herein) discloses a printhead substrate arrangement in which the portion of the substrate that includes the fluid feed slots or channels is made from a ceramic material, while the remaining portion of the substrate arrangement is made by insert molding, i.e. by molding plastic material around the ceramic portion. This arrangement provides for a mounting surface that is flat and stable.
It is desirable to have a printhead substrate (i.e. a mounting substrate to which one or more printhead die can be attached) which costs less to produce. Additionally, it is further beneficial to have a printhead substrate where the widths of the fluid feed slots and the lands between the fluid feed slots are reduced to enable the overall reduction in the size of the corresponding printhead die to be attached. Ceramic is higher in cost than plastic. With ceramic, it is further difficult to provide for desired reduced center-to-center spacing of fluid feed slots, which enable the size the printhead substrate to be reduced. Accordingly, providing a low cost printhead substrate that includes reduced size fluid feed slots and lands there-between when using ceramic may be difficult.
The arrangement of commonly assigned US Published Patent Application 2008/0149024 provides for a mounting surface which is stable as noted above, flat (typically less deviation from flatness than 5 μm per inch), and has a relatively low coefficient of thermal expansion (CTE). The CTE of a material relates the change in temperature to the change in the material's linear dimensions. It is the fractional change in length per degree of temperature change. Depending on the type of ceramic material, the CTE is 4-10 parts per million per degree C. (4-10 ppm/° C.), which is a fairly good match for silicon printhead die commonly used in inkjet applications, having a CTE of about 3 ppm/° C. This is desirable because the stress induced on the silicon printhead die by mounting to the substrate is directly proportional to the difference in the CTE of the silicon die and the material used for the mounting substrate.
As noted above, drawbacks of using the ceramic insert molded substrate approach are cost (ceramic is relatively expensive) and the fact that there are size limitations on the slots widths and pitches (due to the manufacturing limitations of ceramic). The minimum slot to slot pitches typically achieved in a ceramic part made by a low cost powder compaction process are about 1.5 mm (0.7 mm wide slots with 0.8 mm thick walls). Smaller dimensions can be achieved with a ceramic injection molding process, but this will typically increase the cost of the part by about 2-4 times.
A lower cost approach would be to mold the substrate entirely out of plastic. This approach also allows for smaller slot widths and walls than a ceramic part (typically down to a pitch of 1 mm). However, a problem with this approach involves getting sufficient strength in the substrate to provide a stable die mounting surface. Injection molded parts need a uniform wall thickness to have uniform moldability. Since an inkjet device typically needs fairly close spacing of the fluid feed slots, this spacing will determine the maximum wall thickness. Another problem relates to controlling flatness on the die mounting surface of a plastic substrate which can be difficult because of the sink that occurs during the molding process. Although ceramic parts can be made very flat by a low-cost lapping or grinding operation, this cannot be easily done to a plastic part after injection molding, so it is advantageous to mold a plastic die mounting substrate in a way such that the as-molded surface is sufficiently flat. Finally, most plastics have high CTE's (≈25-50 ppm/° C. depending on the type of material) which are much higher than silicon and as a result induce high stresses on the silicon printhead die.
Co-pending U.S. patent application Ser. No. 12/338,211 filed Dec. 18, 2008, incorporated herein by reference, discloses a 2-shot molded printhead substrate which uses the 2nd shot to achieve the close spacing of fluidic slots and the 1st shot to achieve thicker walls in the rest of the substrate to provide sufficient strength to provide a stable die mounting surface. However, what is needed is an arrangement and/or manufacturing method which addresses the CTE and die mounting surface flatness issues noted above, which are commonly encountered when using a plastic substrate in a microelectronic packaging application like inkjet.